The present invention relates to a process for the preparation of caprolactam, wherein
a) a mixture (I) containing 6-aminocapronitrile (xe2x80x9cACNxe2x80x9d) and water is reacted in the gas phase, in the presence of a catalyst, to give a mixture (II) containing caprolactam, ammonia, water, high-boiling components and low-boiling components,
b) ammonia is then removed from the mixture (II) to give a mixture (III) containing caprolactam, water, high-boiling components and low-boiling components,
c) water is then removed from the mixture (III) to give a mixture (IV) containing caprolactam, high-boiling components and low-boiling components, and
d) a solid (V) containing caprolactam is then obtained from the mixture (IV) by crystallization, the proportion by weight of caprolactam in the solid (V) being greater than in the mixture (IV).
Processes for the preparation of caprolactam are generally known.
It is also generally known, for example from Ullmann""s Encyclopedia of Industrial Chemistry, 5th Ed., Vol. A5, VCH Verlagsgesellschaft mbH, Weinheim (Germany), 1986, pages 46-48, or Kirk-Othmer, Encyclopedia of Chemical Technology, 4th Ed., Vol. 4, John Wiley and Sons, New York, 1992, page 836, that caprolactam used for the preparation of polymers must have a purity of 99.9 to 99.94%, the main impurity conventionally being water in an amount of 0.04 to 0.1%. Other impurities must only be present in an amount of at most a few ppm.
Thus caprolactam can be prepared by a Beckmann rearrangement of cyclohexanone oxime with sulfuric acid or oleum. After neutralization of the resulting mixture with ammonia, the caprolactam can be obtained from the ammonium sulfate formed as a by-product by extraction with an organic solvent.
Depending on the processes for the preparation of the educts used to prepare the cyclohexanone oxime, such as cyclohexanone and hydroxylammonium sulfate, and on the oximation and rearrangement conditions, the crude caprolactam obtained by a Beckmann rearrangement contains different types and amounts of impurities. Typical impurities in crude caprolactam prepared by a Beckmann rearrangement are C-methylcaprolactams, 6-methylvalerolactam and n-pentylacetamide.
Various processes are described for the purification of crude caprolactam obtained by a Beckmann rearrangement.
According to DE-A-1253716, the crude caprolactam can be purified by hydrogenation in suspension, in the presence of a catalyst and with the addition of an acid.
According to DE-A-1253716, the crude caprolactam can be purified by hydrogenation in suspension, in the presence of a catalyst and with the addition of a base.
DD-A-75083 describes a process for the purification of crude caprolactam in which the crude caprolactam is first distilled and then dissolved in an organic solvent, hydrogenated in the presence of a catalyst and then treated with an ion exchanger.
According to EP-A-411455, the important characteristic quality features of caprolactam can be preserved by hydrogenating the crude caprolactam continuously in a liquid phase process.
Crude caprolactam obtained by the hydroformylation of 3-pentenoic acid and/or its esters to give 5-formylvaleric acid (esters) as main products and 4- and 3-formylvaleric acid (esters) as by-products, separation of this (these) branched formylvaleric acid (esters) by extraction (WO 97/02228) or distillation (WO 97/06126), aminating hydrogenation of 5-formylvaleric acid (esters) to 6-aminocaproic acid (esters) and/or 6-aminocaproic acid amide, and cyclization of 6-aminocaproic acid (esters) or 6-aminocaproic acid amide, contains other typical impurities.
Thus it is known e.g. from WO 99/48867, Example 1, to crystallize crude caprolactam obtained from 5-formylvaleric acid esters, according to WO 98/37063, Example 9, from mixtures of 6-aminocaproic acid, 6-aminocaproic acid amide and corresponding oligomers, by the addition of 10% by weight of water. This crude caprolactam, from which high-boiling and low-boiling components were not separated before crystallization, contained 6345 ppm of N-methylcaprolactam, 100 ppm of 5-methylvalerolactam, 78 ppm of valeramide and other impurities. The crude caprolactam/water melt was homogenized at 50xc2x0 C. and then cooled to 30xc2x0 C. The crystals which precipitated out were filtered off and washed 2 to 3 times with aqueous caprolactam. The 5-methylvalerolactam and valeramide contents were reduced to 1 ppm and the N-methylcaprolactam content to 51 ppm. 33.7 g of pure lactam were obtained from 73.6 g of crude lactam (caprolactam yield: 45.8%). The characteristic of the volatile bases (VB) was only achieved by a second crystallization. If high-boiling and low-boiling components were separated from the crude caprolactam before crystallization, according to WO 99/48867, Example 3, the caprolactam yield after crystallization was 52%.
It is further known from WO 99/65873 selectively to adsorb caprolactam from mixtures with 4-ethyl-2-pyrrolidone, 5-methyl-2-piperidone, 3-ethyl-2-pyrrolidone and 3-methyl-2-piperidone or octahydrophenazine on adsorbents like activated carbon, molecular sieves or zeolites to give highly pure caprolactam after desorption. This separation of caprolactam can be followed by crystallization from the melt or crystallization from a solvent.
It is further known to purify, by crystallization, crude caprolactam which, starting from 6-aminocapronitrile, is first hydrolyzed with water to 6-aminocaproic acid, according to WO 98/37063, claim 8. Water and ammonia formed by hydrolysis are then separated off, the 6-aminocaproic acid formed is cyclized and the crude caprolactam obtained is crystallized according to WO 99/48867.
Caprolactam can also be obtained by reacting ACN with water in the liquid phase, in the presence or absence of a catalyst, with the release of ammonia.
In addition to caprolactam, water, ammonia and optionally another liquid diluent, the mixture obtained in this reaction contains impurities boiling above caprolactam (xe2x80x9chigh-boiling componentsxe2x80x9d) and impurities boiling below caprolactam (xe2x80x9clow-boiling componentsxe2x80x9d).
It is known from the Example in U.S. Pat. No. 496,941 that, after the separation of water, solvent, ammonia, low-boiling component and high-boiling component from a mixture obtained by reacting ACN with water and solvent, a crude caprolactam is obtained with a purity of 99.5%.
Other methods of purification are described for a crude caprolactam obtained from ACN in the liquid phase since the impurities in this type of crude caprolactam is [sic] markedly different from those in a crude caprolactam obtained by other processes, as described in U.S. Pat. No. 5,496,941.
In a first step, according to U.S. Pat. No. 5,496,941, ACN is converted to caprolactam in the liquid phase, low-boiling components, water, ammonia and optionally other solvents are simultaneously separated off, high-boiling components are separated off to give a crude caprolactam with a purity of 99.5%, this crude caprolactam is hydrogenated in the presence of a catalyst, the product obtained is treated with an acidic ion exchanger or sulfuric acid and the resulting product is distilled in the presence of a base.
WO 96/20923 discloses a method of purifying crude caprolactam originating from the liquid phase cyclization of 6-aminocapronitrile with water in the presence of a solvent and heterogeneous catalysts. In this case, crude caprolactam is first hydrogenated, then treated with acidic agents and finally distilled in the presence of alkali. The disadvantage of this method of purification is that three separate reaction steps are required to prepare pure caprolactam.
The cyclization of 6-aminocapronitrile in the gas phase in the presence of water and a catalyst, for example as described in EP-A-659 741, WO 96/22974, DE 19632006, WO 99/47500 or WO 99/28296, gives a crude caprolactam in which the typical impurities are different from those in a crude caprolactam obtained by another process. Examples of typical impurities in a crude caprolactam obtained from ACN in the gas phase are cyanoalkyl- and aminoalkyl-substituted caprolactam derivatives and tetrahydroazepine derivatives, such as N-cyanopentylhexamethyleneimine, N-cyanopentylcaprolactam and N-aminohexylcaprolactam. In pure caprolactam, these impurities contribute to a degradation of the quality characteristics generally known for caprolactam, for example from Ullmann""s Encyclopedia of Industrial Chemistry, 5th Ed., Vol. A5, VCH Verlagsgesellschaft mbH, Weinheim (Germany), 1986, pages 46-48, or Kirk-Othmer, Encyclopedia of Chemical Technology, 4th Ed., Vol. 4, John Wiley and Sons, New York, 1992, page 836, such as the values for the free and volatile base [sic] and UV characteristics.
It is an object of the present invention to provide a process which makes it possible to prepare, in high purity and in a technically simple and energy-saving manner, caprolactam which has been obtained from ACN in the gas phase.
We have found that this object is achieved by the process defined at the outset.
In step a), a mixture (I) containing 6-aminocapronitrile, water and optionally liquid diluent is converted in the gas phase, in the presence of a solid which promotes the reaction catalytically, to a mixture (II) containing caprolactam, ammonia, water, optionally liquid diluent, high-boiling components and low-boiling components.
The ACN required for step a) can be obtained from adipodinitrile, as is generally known from Ullmann""s Encyclopedia of Industrial Chemistry, 5th Ed., Vol. A5, VCH Verlagsgesellschaft mbH, Weinheim (Germany), 1986, page 46, FIG. 8.
Particularly appropriate here is the partial catalytic hydrogenation of adipodinitrile in the presence of ammonia as solvent and e.g. in the presence of rhodium on magnesium oxide (U.S. Pat. No. 4,601,859), Raney nickel (U.S. Pat. No. 2,762,835, WO 92/21650) or nickel on aluminum oxide (U.S. Pat. No. 2,208,598) as a suspension catalyst or Cuxe2x80x94Coxe2x80x94Zn spinel (DE-B-954416, U.S. Pat. No. 2,257,814) or iron (DE-A-42 35 466) as a fixed bed catalyst, or a process according to U.S. Pat. No. 2,245,129, U.S. Pat. No. 2,301,964, EP-A-150295 or FR-A-2 029 540, or a process described in U.S. Pat. No. 5,496,941.
The adipodinitrile required for this reaction is prepared industrially, e.g. by the double hydrocyanation of butadiene in the presence of nickel-containing catalysts, and is commercially available, e.g. from Aldrich-Chemie Gesellschaft mbH and Co. KG, Steinheim, Germany. The conversion of the mixture (I) to the mixture (II) can be carried out according to EP-A-659 741, WO 96/22974, DE 19632006, WO 99/47500 or WO 99/28296 for example.
The reaction can preferably be carried out in the gas phase at temperatures generally of 200 to 550xc2x0 C., preferably of 250 to 400xc2x0 C.; the pressure ranges generally from 0.01 to 10 bar and is preferably atmospheric pressure, it being necessary to ensure that the reaction mixture is predominantly gaseous under the conditions used.
The catalyst loads are usually 0.05 to 2, preferably 0.1 to 1.5 and particularly 0.2 to 1 kg of 6-aminocapronitrile per liter of catalyst volume per hour.
The reaction can be carried out batchwise or, preferably, continuously.
Suitable reactors are advantageously those which are generally known for gas phase reactions on moving or stationary solid catalysts. It is preferred to use a fluidized bed reactor or, preferably, a fixed bed reactor such as a tray reactor, especially a tubular reactor. Combinations of such reactors are also possible.
The amount of water used is generally 1 to 50, preferably 1 to 10 mol per mol of ACN.
The mixture (I) can also contain other organic compounds which are gaseous under the reaction conditions, such as alcohols, amines or aromatic or aliphatic hydrocarbons.